This invention relates to so-called froth flotation methods and apparatus in which air in the form of bubbles is delivered to a slurry or multi-phase fluid for causing separation by flotation of the flotable phase from the non-flotable phase in the slurry or fluid.
The froth flotation methods of this invention represent improvements over the conventional methods, such as those disclosed for example in U.S. Pat. Nos. 2,713,477 and 3,491,880, in which a quantity of slurry of particulate material having a flotable phase and a non-flotable phase is provided in a tank, air in the form of bubbles is delivered to and mixed with the slurry to form a froth comprising a mixture of air bubbles and the flotable phase from the slurry, the froth is withdrawn from the tank through an upper outlet port in the tank, and the nonflotable phase is withdrawn through a lower outlet port in the tank. These conventional froth flotation methods and apparatus for practicing them have been widely used for the separation of metallic and non-metallic minerals, and other industrial processes such as purification of food products, de-inking of paper pulp and treating industrial wastes. In some applications, the flotable phase of the slurry is the desired product and the non-flotable is a less desired by-product (and in some cases a waste by-product), and in other applications, the reverse is true.
The performance of the froth flotation apparatus in effecting separation of the flotable from the non-flotable phase of a slurry is measured by two factors; namely, rate and selectivity, which are generally inversely related to each other. The rate of separation is a measure of the quantity or mass of the froth of flotable phase removed from the slurry over a given period of time while selectivity is a measure of the quality of the froth removed. Rate is typically expressed as tons per hour, and selectivity is the ratio of the percent of flotable material in the froth to the percent of non-flotable material in the froth. As indicated previously rate and selectivity are inversely related so that increases in the rate of separation have the undesired but unavoidable affect of reducing the selectivity (or quality) of the removed material. In those applications in which the desired product is the flotable phase, increased rates have the unintended result of greater inclusion of the undesired non-flotable phase along with the desired flotable phase in the froth, which then requires additional downstream separation apparatus to achieve the desired froth purity. In those applications in which the desired product is the non-flotable phase, increased rates have the unintended result of removing a portion of the desired non-flotable phase along with the undesired flotable phase from the slurry, again requiring additional downstream separation apparatus to recover the "lost" desirable non-flotable material.
To date, attempts to maximize the rate and selectivity of froth removal in froth flotation machinery have been principally directed at providing a series of so-called froth flotation cells so arranged and sized relative to each other as to give the most cost effective balance between the degree of recovery of the desirable material, the purity of the separated products, and the increased capital and maintenance costs for the machinery. In such multi-cell machinery, the cells are connected in serial flow arrangement, each cell operating to remove a portion of the flotable phase at a predetermined rate and selectivity, and with each downstream cell thus receiving slurry for treatment at a lower concentration of the flotable phase than the cells upstream thereof. As shown for example in U.S. Pat. No. 3,491,880 improvements have also been made directed to increasing the volume of the air delivered to the slurry, the efficiency of the equipment for delivering the air, and the mixing action of the air bubbles in the slurry to form the froth. While machinery of this design has been generally satisfactory as evidenced by its wide usage in a number of industries, neither this froth flotation machine nor the other conventional machines embody a recognition of the physical phenomenon occurring between the air bubbles and the flotable phase for effecting its flotation as froth, much less a recognition of which factors may be controlled (and how to control such factors) to enhance the flotation phenomenon for increasing both the rate and the selectivity of the froth flotation.